PowerConnect Application Note #5 November 2003 How Much Broadcast and Multicast Traffic Should I Allow in My Network? This Application Note relates to the following Dell PowerConnect products: PowerConnect 33xx PowerConnect 52xx Abstract Broadcast and multicast traffic perform valuable roles in terms of network discovery and content delivery, but too much of either can have an adverse effect on performance. Excessive amounts of broadcast or multicast traffic not only waste bandwidth, but also degrade the performance of every device attached to the network. This application note discusses the steps network managers can take to minimize the effects of broadcast and multicast traffic without compromising application functionality or performance. Applicable Network Scenarios The diagram below depicts a corporate network carrying multiprotocol traffic. While there is no universal threshold that determines how much broadcast or multicast traffic is too much, this is definitely a network at risk. Each of the various protocols on the network potentially can degrade the performance of all stations attached to the network. Corporate WAN 192.168.157.0/24 Router IP-directed broadcasts 192.168.1.0/24 Dell PowerConnect 3348 Netware server SAP broadcasts ff:ff:ff:ff:ff:ff Exchange server Netbios broadcasts 192.168.1.255 Streaming video server IP multicast 224.0.0.0 1
The Microsoft Exchange server uses Netbios broadcasts for host discovery, while the Novell Netware server transmits IPX/SPX service access point (SAP) broadcasts. Both Netbios and IPX/SPX are chatty protocols, meaning each transmits large amounts of broadcast traffic. A router connects this segment to other networks. Routers may forward IP-directed broadcasts, again consuming bandwidth. All these broadcasts tie up system resources as well as consuming network bandwidth. Every node (PC, server, printer, or any other network-attached device) in a given broadcast domain must process each broadcast frame it receives. When a node receives a broadcast, it generates an interrupt. In turn, each interrupt consumes some amount of processing time by the node. Multicast traffic is somewhat more benign, though it too can affect network performance. The streaming video server in this diagram uses IP multicast for traffic delivery. While multicast reduces bandwidth consumption by sending traffic only to subscribers of a given group, multicast-enabled applications tend to be bandwidth-intensive. Measure first, then manage Before applying any traffic management commands to switches, it is essential to understand how much unicast, multicast, and broadcast traffic exists in the network. The amount of broadcast and multicast traffic on a given network may be minuscule, or it may overload the existing infrastructure. The adage you can t manage what you can t measure applies here. Because Dell PowerConnect switches keep track of all traffic they handle, it is relatively simple to determine the amount of multicast vs. non-multicast traffic in the network. Every interface of every Dell PowerConnect switch has counters that measure the number of frames, bytes, and errors it sees. These counters are broken down by transmit and receive traffic, and also by the type of traffic handled multicast, unicast, or broadcast. Here is the command-line interface sequence to view counters for interface 23 of a Dell PowerConnect 3348. The same command applies on a PowerConnect 3324: console# show interface counters ethernet 1/e23 Port InOctets InUcastPkts InMcastPkts InBcastPkts ---------------- ---------- ----------- ----------- ----------- 1/e23 1488159 1 1488095 Port OutOctets OutUcastPkts OutMcastPkts OutBcastPkts ---------------- ---------- ------------ ------------ ------------ 1/e23 6400 0 100 Alignment Errors: 0 FCS Errors: 0 Single Collision Frames: 0 Multiple Collision Frames: 0 SQE Test Errors: 0 Deferred Transmissions: 0 Late Collisions: 0 Excessive Collisions: 0 Internal MAC Tx Errors: 0 Carrier Sense Errors: 0 Oversize Packets: 0 Internal MAC Rx Errors: 0 Symbol Errors: 0 Received Pause Frames: 0 Transmitted Pause Frames: 0 2
In the instance above, the interface has received 1 unicast frame and 1,488,095 multicast (what type of frames? Multicast?) frames. Therefore, in this case the ratio of unicast to inbound multicast traffic is 1:1,488,095. Here is a similar example for viewing interface statistics from interface 24 of a Dell PowerConnect 5224: Vty-0#show interfaces counters ethernet 1/12 Ethernet 1/12 Iftable stats: Octets input: 1564024419, Octets output: 649097 Unicast input: 7429, Unicast output: 4762 Discard input: 0, Discard output: 0 Error input: 0, Error output: 0 Unknown protos input: 0, QLen output: 0 Extended iftable stats: Multi-cast input: 75630, Multi-cast output: 3981 Broadcast input: 1115761, Broadcast output: 0 Ether-like stats: Alignment errors: 0, FCS errors: 0 Single Collision frames: 0, Multiple collision frames: 0 SQE Test errors: 0, Deferred transmissions: 0 Late collisions: 0, Excessive collisions: 0 Internal mac transmit errors: 0, Internal mac receive errors: 0 Frame too longs: 0, Carrier sense errors: 0 Symbol errors: 0 RMON stats: Drop events: 0, Octets: 1564673940, Packets: 1207574 Broadcast pkts: 1115763, Multi-cast pkts: 79614 Undersize pkts: 0, Oversize pkts: 0 Fragments: 0, Jabbers: 0 CRC align errors: 0, Collisions: 0 Packet size <= 64 octets: 124645, Packet size 65 to 127 octets: 37922 Packet size 128 to 255 octets: 3740, Packet size 256 to 511 octets: 20599 Packet size 512 to 1023 octets: 29, Packet size 1024 to 1518 octets: 1020639 In this example with a PowerConnect 5224, the interface has received 7,429 unicast frames, 75,630 multicast frames, and 1,115,761 broadcast frames. The ratio of unicast to multicast in this case is about 1:10, but the broadcast frame count dwarfs both the unicast and multicast totals. Interface counters have two limitations when it comes to determining multicast-vs.-non-multicast traffic breakdowns. First, counters are cumulative. They present information about traffic over time, not real-time rates. So, for example, it may appear that a switch interface handled about the same amounts of multicast and unicast traffic, even though hours or days have passed between the times when multicast and unicast traffic were active. The best way to keep track of traffic statistics is to import them into a network management system such as one using simple network management protocol (SNMP). Dell PowerConnect switches support standards-based SNMP and remote monitoring (RMON), an extension to SNMP intended specifically for device and traffic management. By tracking traffic over time, it is possible to get a sense of longer-term trends of multicast vs. non-multicast usage. This sort of long-term observation is essential in capacity planning the science of determining network resource requirements based on measurements of past and current trends. A second problem with counters is that they are accurate only as of the last system reset, the last time the counters were cleared, or the last time the counter wrapped (reached its maximum value, at which time the counter reverts to 0 and starts counting up again). Here again, importing counter statistics into 3
an external network management system will provide the most accurate picture of traffic statistics over time. Technology Background Dell PowerConnect switches offer several mechanisms for reducing the overhead associated with broadcast and multicast traffic. These mechanisms include IEEE 802.1Q VLANs, storm control, and Internet Group Management Protocol (IGMP) snooping. VLANs logically segment network nodes into a common broadcast domain isolated from other VLANs and physical segments. Thus, even if broadcast or multicast traffic is heavy on one VLAN, it can be isolated from the rest of the network. If isolating a segment within a VLAN is not an option, network managers can implement another mechanism known as storm control. Storm control regulates the rate at which switches forward broadcast and multicast traffic. Dell PowerConnect switches allow network managers to specify a maximum rate (given in packets per second) for broadcast traffic. To control rates of multicast traffic, the PowerConnect switches support a feature called IGMP snooping. Normally, a switch receiving a multicast frame will flood it that is, transmit the frame on all ports in the VLAN associated with the interface that injected it. With IGMP snooping enabled, the switch floods ports only once. After the initial flood, the switch intercepts IGMP traffic sent between an IGMP router and hosts wishing to join a multicast group. Once a switch determines which hosts joined the multicast group, it forwards all subsequent multicast traffic only to those ports to which the hosts are attached. Proposed Solution Overview Network managers can address the performance degradation associated with the excessive broadcast and multicast traffic by taking the following steps: 1. Disable IP-directed broadcasts on the router 2. Enable storm control on Dell PowerConnect switches 3. Verify that storm control is operating correctly 4. Enable IGMP snooping on Dell PowerConnect switches 4
Typical Network Designs IP-directed broadcasts Corporate WAN 192.168.157.0/24 Router IP-directed broadcasts 192.168.1.0/24 Dell PowerConnect 3348 (storm control, IGMP snooping) Netware server SAP broadcasts ff:ff:ff:ff:ff:ff (250 fps max) Exchange server Netbios broadcasts 192.168.1.255 (250 fps max) Streaming video server IP multicast 224.0.0.0 IGMP snooping enabled With IP-directed broadcasts disabled on the router, all external traffic destined for 192.168.1.255 will be filtered and dropped, keeping out broadcast storms. IGMP snooping restricts all multicast traffic to ports participating in an IGMP group. Storm control limits high-overhead protocols to an arbitrary rate; in this case, broadcasts are getting throttled at 250 packets per second. Step-By-Step Instructions The following configuration guidelines are compatible with Dell PowerConnect 33xx and 52xx series switches. The Cisco router configurations are compatible with most Cisco routers running IOS. 1. Disable IP-directed broadcasts on the router. Router1> enable Router1# configure terminal Router1(config)# no ip directed-broadcast Router1(config)# exit 2. Enable storm control on Dell PowerConnect switches. For PowerConnect 33xx series: console# configure console(config)# port storm-control enable broadcast fastethernet console(config)# port storm-control rate fastethernet 250 NOTE: Storm control is not supported in PowerConnect 52xx switches. 5
3. Verify that storm control is operating correctly. For PowerConnect 33xx series: console# show port storm-control Port Unknown Broadcast Multicast Rate[Packets/sec] ------------------ --------- ----------- ----------- ------------------- FastEthernet 1/e1 Disabled Enabled Enabled 250 FastEthernet 1/e2 Disabled Enabled Enabled 250 FastEthernet 1/e3 Disabled Enabled Enabled 250 FastEthernet 1/e4 Disabled Enabled Enabled 250 FastEthernet 1/e5 Disabled Enabled Enabled 250 FastEthernet 1/e6 Disabled Enabled Enabled 250 FastEthernet 1/e7 Disabled Enabled Enabled 250 FastEthernet 1/e8 Disabled Enabled Enabled 250 FastEthernet 1/e9 Disabled Enabled Enabled 250 FastEthernet 1/e10 Disabled Enabled Enabled 250 FastEthernet 1/e11 Disabled Enabled Enabled 250 FastEthernet 1/e12 Disabled Enabled Enabled 250 FastEthernet 1/e13 Disabled Enabled Enabled 250 FastEthernet 1/e14 Disabled Enabled Enabled 250 FastEthernet 1/e15 Disabled Enabled Enabled 250 FastEthernet 1/e16 Disabled Enabled Enabled 250 FastEthernet 1/e17 Disabled Enabled Enabled 250 FastEthernet 1/e18 Disabled Enabled Enabled 250 FastEthernet 1/e19 Disabled Enabled Enabled 250 FastEthernet 1/e20 Disabled Enabled Enabled 250 FastEthernet 1/e21 Disabled Enabled Enabled 250 FastEthernet 1/e22 Disabled Enabled Enabled 250 FastEthernet 1/e23 Disabled Enabled Enabled 250 FastEthernet 1/e24 Disabled Enabled Enabled 250 4. Enable IGMP snooping on Dell PowerConnect switches. For PowerConnect 33xx and 52xx series: console(config)# ip igmp snooping To discover which multicast addresses a PowerConnect switch has learned, use this command: console# show ip igmp snooping groups Conclusion Excessive broadcast and multicast traffic can seriously degrade network performance. By following the configuration examples given here and disabling IP-directed broadcasts on edge routers, it is possible to mitigate the effects of excessive broadcast and multicast traffic. Information in this document is subject to change without notice. 2003 Dell Inc. All rights reserved. This Application Note is for informational purposes only, and may contain typographical errors and technical inaccuracies. The content is provided as is, without express or implied warranties of any kind. Trademarks used in this text: Dell, the DELL logo, and PowerConnect are trademarks of Dell Inc.. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and names or their products. Dell Inc. disclaims any proprietary interest in trademarks and trade names other than its own. 6